How do I post to a private Facebook group from my web application

I have this web application and we want to post certain events to a closed Facebook group from this web application automatically by calling an API than manually click of 'Share' button, how could we do that?

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There are suspicious activities of some Facebook users occurring in my app involving the possibility of a user being logged into another user account. Is there a Facebook channel to see if these suspicious activities generated any kind of alert on their platform?

Facebook sharing using feed or share has a success call back and can tell if user shared or not, but when using iframe it allways says that the user canceled the dialog {error_code: 4201, error_message: "User+canceled+the+Dialog+flow"}and cant tell if the user shared or not, but same function without the use of iframe is working properly